Heating pads are often used to keep individuals or certain muscles of an individual warm. Typically, a heating pad includes first and second sheets of cloth material having a heating structure captured therebetween. The heating structure includes a heating core wire connectable to an electrical power source through a controller. The controller allows a user to vary the magnitude of the electrical power provided to the heating core wire, and hence, to control the heat dissipated by the heating core wire and the temperature of the heating pad.
The heating core wire is contained in an insulation sheath that is protected by a woven screen. In normal use, it is intended for the heating pad to be spread out over a large surface area in order for the whole surface to dissipate the heat generated by the heating core wire. It can be appreciated that the maximum temperature reached by the heating core wire must be below the melting point of the insulation. However, the risk of a heating pad overheating may be significant in those situations when the heating core wire is electrically coupled to the electrical power source and the heating pad is folded onto itself so as to prevent the heat generated by the heating core wire to escape. In such a situation, a first layer of the folded heating pad may heat an adjacent layer (and vice versa) to such point as to cause the heating pad to reach a temperature that is capable of starting a fire. It is noted, however, that the insulation about the heating core wire must melt before the cloth layers of the heating pad begin to char and a fire starts.
In order to prevent the possibility of a fire starting due to the overheating of the heating pad, most heating pads incorporate a protective circuit. By way of example, Weiss et al., U.S. Pat. No. 5,422,461 discloses an electrical feedback safety circuit and a semiconductor switching system to control power to a heating element of the Positive Temperature Co-efficient “PTC” type that requires a safe operation condition in the event of an open or short circuit. An integrated circuit is used to signal a solid state switch to time the on and off proportion of the AC electric power to a flexible PTC heating element in order to control the temperature. Since the PTC element has the property of increased electrical resistance with increased temperature, the natural effect of increasing temperature is to throttle down and limit the current draw. The ability to control the temperature of the heater, by current control or time proportioned power control is improved. The power control level is affected externally by a heat scale setting via up-down key pad or rotary potentiometer and internally by the feedback safety circuit.
While functional for its intended purpose, the feedback safety circuit disclosed in the '461 patent has certain disadvantages. For example, the circuit includes two triacs which are both controlled by a single microprocessor. In the event the microprocessor fails, both triacs will be rendered inoperable. Further, triacs are relatively costly components. Incorporating two triacs into the design of the feedback safety circuit significantly increases the costs associated with the heating pad. As such, it is highly desirable to provide a temperature control and safety protection device for a heating pad that overcomes the disadvantages of the prior art.
Therefore, it is a primary object and feature of the present invention to provide a heating pad with a temperature control and safety protection device that has dual independent electronic hardware circuitries for preventing overheating of the heating pad.
It is a further object and feature of the present invention to provide a heating pad with a temperature control and safety protection device that is less expensive to manufacture than prior devices.
It is a still further object and feature of the present invention to provide a heating pad with a temperature control and safety protection device that shuts off the heating pad if a first, upper temperature limit is reached and that blows a fuse if a second, higher temperature limit is reached.
In accordance with the present invention, an improvement for heating pad having a heating element is provided. The improvement includes a control circuit operatively connected to the heating element for providing a predetermined level of power thereto. An isolation device is operatively connecting the control circuit to a power source. The control circuit terminates power to the heating element in response to the heating element exceeding a first temperature limit. The isolation device isolates the control circuit from the power source in response to the heating element exceeding a second temperature limit.
The second temperature limit is greater than the first temperature limit. The control circuit includes a central processing unit and a comparator interconnecting the heating element to central processing unit. The comparator proves a signal to the central processing unit in response to the heating element exceeding the first temperature limit. It is contemplated for the isolation device is a fuse. The comparator is interconnected to the fuse. The comparator causes the fuse to blow in response to the temperature of the heating element exceeding the second temperature limit. The control circuit also includes a switch interconnecting the central processing unit to the heating element. The central processing unit opens the switch in response to the heating element exceeding the first temperature limit so as to disconnect the supply of power to the heating element. The switch may be a triac.
In accordance with a further aspect of the present invention, a heating pad is provided. The heating pad includes a heating element and a control circuit operatively connected to the heating element for providing a predetermined level of power thereto. An isolation device interconnects a power source to the control circuit. The isolation device has a first open configuration preventing transmission of electrical power therethrough and a second closed configuration allowing transmission of electrical power therethrough. The control circuit terminates power to the heating element in response to the heating element exceeding a first temperature limit. The isolation device moves from the closed configuration to the open configuration in response to the heating element exceeding a second temperature limit.
The second temperature limit is greater than the first temperature limit. The control circuit includes a central processing unit and a comparator interconnecting the heating element to central processing unit. The comparator provides a signal to the central processing unit in response to the heating element exceeding the first temperature limit. It is contemplated for the isolation device to be a fuse. The comparator is interconnected to the fuse. The comparator causes the fuse to blow in response to the temperature of the heating element exceeding the second temperature limit. The control circuit also includes a switch interconnecting the central processing unit to the heating element. The central processing unit opens the switch in response to the heating element exceeding the first temperature limit so as to disconnect the supply of power to the heating element. The switch may be a triac.
The heating element includes a heating conductor operatively connected to the control circuit and a negative temperature coefficient layer surrounding the heating conductor. A positive temperature coefficient sensor conductor extends about the negative temperature coefficient layer and is operatively connected to the control circuit.
In accordance with a still further aspect of the present invention, a heating pad is provided. The heating pad includes a heating conductor and a negative temperature coefficient layer surrounding the heating conductor. A positive temperature coefficient sensor conductor having a resistance extends about the negative temperature coefficient layer. A control circuit is operatively connected to the heating conductor for supplying power thereto. An isolation device interconnects the control circuit to a power source. The isolation device has a first open configuration preventing transmission of electrical power therethrough and a second closed configuration allowing transmission of electrical power therethrough. The control circuit terminates power to the heating conductor in response to the resistance of the positive temperature coefficient sensor conductor exceeding a first threshold. The isolation device moves from the closed configuration to the open configuration in response to the shorting of the positive temperature coefficient sensor conductor to the heating conductor.
The control circuit includes a central processing unit and a comparator interconnecting the heating conductor to central processing unit. The comparator provides a signal to the central processing unit in response to the resistance of the positive temperature coefficient sensor conductor exceeding the first threshold. The isolation device may be a fuse. The comparator is interconnected to the fuse. The comparator causes the fuse to blow in response to the shorting of the positive temperature coefficient sensor conductor to the heating conductor. The control circuit also includes a switch interconnecting the central processing unit to the heating conductor. The central processing unit opens the switch in response to the resistance of the positive temperature coefficient sensor conductor exceeding a first threshold. It is contemplated for the switch to be a triac.